1. Field of the Invention
The present invention relates to a honeycomb structure. More particularly, it relates to a honeycomb structure in which a ring crack is not easily generated.
2. Description of Related Art
Heretofore, in an exhaust system of an exhaust gas, there has been mounted an exhaust gas purification device including a diesel particulate filter (DPF), a catalyst body, or the like. The DPF is a filter to trap a particulate matter (PM) mainly composed of soot. The catalyst body purifies harmful substances such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) included in the exhaust gas. Furthermore, a honeycomb structure is used as a catalyst carrier constituting this catalyst body or the DPF.
One of causes for a failure of the exhaust gas purification device is, for example, generation of a ring-like crack in the honeycomb structure. Such a crack generated in the honeycomb structure will be referred to as “the ring crack”. As shown in FIG. 8, this ring crack is a crack (a ring crack 50) formed substantially perpendicularly to an extending direction of cells 2 in a honeycomb structure 200. Furthermore, the ring crack 50 is formed to extend in a circumferential direction in an outer periphery of the honeycomb structure. FIG. 8 is a perspective view schematically showing a state where the ring crack is generated in the conventional honeycomb structure.
It is considered that a usual factor of this ring crack is generation of a tensile stress in the whole length direction (the cell extending direction) of the honeycomb structure. The tensile stress is a stress generated by a difference between a thermal expansion coefficient of a ceramic material which constitutes the honeycomb structure and a thermal expansion coefficient of a metal can body which stores this honeycomb structure. This tensile stress is generated when the honeycomb structure is placed in the environment where heating and cooling are repeated. There is a tendency that this tensile stress increases as a ratio (A/B) is large in which A is a length (the whole length (A)) of the honeycomb structure in the cell extending direction, and B is a diameter (the outer diameter (B)) of the honeycomb structure in a cross section perpendicular to the cell extending direction. Therefore, the larger the ratio A/B is, the more easily the ring crack is generated. Here, the whole length (A) is “the length in the cell extending direction” of the honeycomb structure. Furthermore, the outer diameter (B) is the diameter in “the cross section perpendicular to the cell extending direction” of the honeycomb structure.
Thus, as a method of inhibiting the generation of the ring crack, a method of enlarging the outer diameter to decrease the ratio A/B is known. Furthermore, there has been suggested a honeycomb structure in which the ratio A/B is in a predetermined range (e.g., see Patent Document 1).
[Patent Document 1] JP-A-H09-299811